Answer: Water is present in the air. When the air is compressed all that water comes with it. Temperature and humidity determine how much water there is in the air. Click here to get a fuller explanation of water and other contaminants in the air lines. However, if water in your air lines in something new, then there is a failure somewhere in your air systems. A filter drain may be plugged or an auto-drain is malfunctioning. Another common problem is that when running a compressor hot (meaning above 50% duty cycle) the water in the air lines is vapour and doesn’t condense in the tank (the 1st water separator in any air system). This hot vapour is carried through the air lines and condenses on any cool surface. This water then collects all through the air lines and builds up. When some process demands large amounts of air that built up water gets carried through and seems like it is a sudden problem, when all along it has been building up. The solution is better water separation at the source (the compressor) with a refrigerated air dryer and separator/filters with auto-drains.